1. Field of the Invention
The present invention relates to an extender for a universal serial bus (USB) that effectively extends USB cable length above its current approximate five meter limit. Specifically, the invention relates to USB transmit and receive hubs which respectively include non-USB data transport interfaces together with suitable USB-to-non-USB translators, that effectively extend the reach of USB devices.
2. Description of the Related Art
In the evolution of personal computers, one significant recent development is the introduction of universal serial bus (USB). USB is an interface that supports serial data transfers between a USB host computer (sometimes referred to as the “root node” or “root hub”) and USB-capable peripherals. The host serves as the master of the bus, and the peripherals perform as slaves connected either directly to the host or indirectly through hubs, in a tiered star topology with one of the hubs at the center of each star.
Without USB, a personal computer requires a separate interface, with specialized electrical, mechanical and software connections, so as to be able to connect to each individual peripheral. Thus, without USB, a personal computer requires separate interfaces for its keyboard, monitor, modem, printer, microphone, joy stick, mouse, scanner, and so forth. With USB, on the other hand, USB-capable peripherals can be connected directly to USB hubs on the bus, without the need for any specialized software interfaces or mechanical or electrical interfaces.
USB is described with specificity in “Universal Serial Bus Specification”, Version 1.1, Sep. 23, 1998, Compaq Computer Corporation, et al., the contents of which are incorporated herein by reference. Generally speaking, on any USB system, there is exactly one USB host which is typically a personal computer built around USB-capable motherboards and equipped with USB software. The host acts as master of the bus, acknowledging attachment and removal of peripherals, initiating enumeration processes and all subsequent USB transactions on the bus, collecting status and activity statistics, and controlling electrical interface between the host and USB peripherals. USB peripherals act as slaves on the bus and are of two types: “hubs” and “functions”. A “hub” typically consists of a hub controller and a repeater, and usually converts a single upstream attachment point into multiple downstream attachment ports. “Functions” are PC peripherals like keyboards, joy sticks, cameras, printers and the like. A “Function” can be self-powered or bus-powered, meaning it derives its power from the USB bus; likewise, a “hub” can be self-powered or bus-powered, to provide power to downstream devices (which may be hubs or functions) attached to its ports.
USB cabling, which connects the host with peripherals (i.e., both hubs and functions) consists of four wires including a twisted pair for D+ and D− used for actual data transfer as well as a power wire and a ground wire. The cabling includes connectors that differ at the upstream and downstream sides, so as to prevent non-USB compliant connections and so as to ease use by preventing mis-cabling: a USB “A” type plug on the upstream side toward the USB host, and a USB “B” type plug toward the downstream side (towards the peripherals).
While USB provides good interconnectivity between the host and its peripherals, a limitation on the length of USB cabling presents a practical limitation on long distance data transfer. Specifically, because of electrical and frequency constraints, USB cable can be no longer than approximately five meters in length. It is possible to circumvent this length limitation to some extent, by end-to-end connection of multiple USB hubs, each of which acts as a repeater, with separate five meter lengths of cable therebetween. There are other practical limitations on such an arrangement, which in any event is expensive and inconvenient. Accordingly, a better solution is needed.